Mulch Calculator — Bags, Cubic Yards and Cost

Work out how much mulch a garden bed or landscaped area needs, in bags, cubic feet, cubic yards or cubic metres — for one bed or several at once, in feet and inches or metres and centimetres.

Created by Ashar PervaizLast updated 24 October 2026

How Much Mulch for a Garden Bed?

Three measurements settle it: how long the bed is, how wide, and how deep you intend to spread. Length times width gives the area, and area times depth gives the volume — with the one wrinkle that depth is quoted in inches while the bed is measured in feet, so the depth has to be divided by twelve before the three numbers can be multiplied together.

Area (sq ft) = length × width
Volume (cu ft) = area × (depth in inches ÷ 12)
Bags = volume ÷ bag size
Cubic yards = volume ÷ 27

A border 20 feet long and 4 feet deep is 80 square feet. At three inches the depth in feet is 0.25, so the volume is 80 × 0.25 = 20 cubic feet. In standard two-cubic-foot bags that is ten bags, and in bulk terms it is 20 ÷ 27, or 0.74 of a cubic yard.

That last figure is the awkward one, and it is worth noticing early. Bulk mulch is sold by the cubic yard, and most domestic beds need an unhelpful fraction of one.

What One Bag Actually Covers

Because mulch is usually bought in bags, the useful coverage figure is per bag rather than per cubic yard. A two-cubic-foot bag spread at a given depth covers an area you can work out by dividing the bag volume by the depth in feet — and because that depth sits underneath, the coverage halves every time the depth doubles.

The third column is the one that changes minds at the checkout. The same hundred square feet takes nine bags at two inches and seventeen at four — the depth you pick is very nearly the whole cost of the job, and it is the input people give least thought to.

Two to Three Inches, and Why More Is Worse

With stone, deeper generally means sturdier. Mulch is the opposite: it has a ceiling as well as a floor, and overshooting causes problems that under-mulching does not.

Below about two inches there is simply not enough material to block light, so weed seeds germinate through it and the main job goes undone. Above about four, the layer starts working against the bed it is meant to help. Light rain is absorbed by the mulch and evaporates before reaching soil. The ground underneath stays cold and airless. Roots, finding moisture in the mulch rather than below it, grow upward into a layer that dries out hard in the first warm spell.

The worst version is mulch heaped against a trunk or stem. Bark that is kept permanently damp rots, and because the damage happens at ground level under a mound, it is usually well advanced before anything visible goes wrong higher up. Whatever depth you choose across the bed, pull it back to leave the base clear.

A spade or a rake marked in fractions gives you a depth like two and a half inches rather than 2.5, and the box above wants the decimal. The mixed number calculator will turn any fraction into one.

Bags or Bulk, and Where Mulch Differs From Stone

A cubic yard is 27 cubic feet, so at two cubic feet a bag it is 13.5 bags — a half-bag remainder that makes bulk and bagged prices annoying to compare. The practical version is that you will buy fourteen and have some over.

The crossover sits lower than it does for aggregates, though, and for a reason worth knowing. Mulch is light, so a volume that would be punishing to move as stone is manageable as bark. Bags stay reasonable further up the scale, and the decision is usually about whether you have somewhere to tip a bulk load rather than whether you can carry it. Below roughly a cubic yard, bags nearly always win; past two or three, a delivery does.

If you are pricing bagged against bulk, the unit conversion calculator will put both on the same footing, and the percentage calculator will turn the gap into a figure worth acting on.

Mulch Disappears, and That Is the Point

Unlike gravel, mulch is consumable. It breaks down into the soil, which is half of what you are paying for — the weed suppression is this season's benefit, the soil improvement is next season's. A bed that looked three inches deep in spring may be nearer an inch and a half by the following one.

This changes how you should measure for a top-up. Rake back what is left, find out how deep it actually is, and calculate for the shortfall rather than for the full depth again. Beds topped up annually without that check climb steadily past the useful range, and arrive at the smothering problem by accident rather than by decision.

Finer materials break down faster than coarse chip, so the rhythm is set by what you spread rather than by the calendar.

Several Beds, and Beds That Are Not Rectangles

Most gardens are a handful of beds rather than one, and the calculator adds them for you — measure each as its own rectangle and let the totals combine. This beats measuring the whole garden and guessing a proportion, because the error in a guess applies to every square foot at once.

Curved borders are best squared off generously: take the longest length and the widest width, accept that you are over-measuring a little, and treat that as part of the allowance rather than as a mistake. Mulch left over has uses. Mulch three bags short on a Saturday afternoon does not.

Why Add Five to Ten Per Cent

Three ordinary losses sit between the volume you calculate and the volume that ends up on the bed. Fresh mulch settles once it takes rain. Some is lost over edges, onto paths and into the barrow. And a bed measured as a clean rectangle almost never is one.

Five to ten per cent covers all three for most jobs, at the lower end for a tidy bed with hard edges and the upper for anything awkward or exposed. The allowance here starts at zero deliberately, so the plain arithmetic is visible first and whatever you add on top is a decision you made rather than a margin somebody buried in the formula.

Mulch is usually laid over ground that has already been dug, edged or levelled, and each of those has its own quantity to order. The other materials calculators work in the same cubic yards and bags this one does.

Mulch Quantity Questions